1. Field of the Invention
The present invention relates to a semiconductor device, a display device, and an electronic appliance each having a light-emitting element with excellent light-emitting characteristics and also relates to a manufacturing method of a semiconductor device.
2. Description of the Related Art
FIG. 16 is a drawing for explaining a structure of a conventional light-emitting element. The conventional light-emitting element has a structure in which a lower electrode 1002, a semiconductor layer 1004 of a first conductivity type, a light-emitting layer 1006, a semiconductor layer 1008 of a second conductivity type, and an upper electrode 1010 are stacked over an insulating film 1000 in this order. When a predetermined potential is given to each of the lower electrode 1002 and the upper electrode 1010, excitons are recombined in the light-emitting layer 1006 and the energy released at the recombination is emitted as light.
In a case of forming a display using a light-emitting element, light needs to be emitted from a top surface of the light-emitting element. When the upper electrode 1010 is formed by using metal, there is a method in which the upper electrode 1010 is formed to have a thickness from 5 to 200 nm so that light is emitted from the top surface. By this method, when the upper electrode 1010 formed of Ni has a thickness of 15 nm, the transmittance of ultraviolet light emitted from the light-emitting element becomes 70% or more (for example, Reference 1: Japanese Published Patent Application No. 2004-221132, 43rd paragraph and 57th paragraph).
According to the aforementioned related art, the transmittance of emitted light can be made 70% or more by decreasing the film thickness of the upper electrode (for example 15 nm). However, when the light-emitting element is used for a display, the light-transmittance of the upper electrode is desired to be 80% or more.
Moreover, as another method, a transparent electrode including ITO, ZnO, or the like may be used as the upper electrode. However, since the transparent electrode has high resistance, the transparent electrode consumes electric power to cause the light-emitting element to have lower luminous efficiency. In addition, when ultraviolet light is emitted using ZnO for the light-emitting layer, the use of ZnO for the upper electrode causes the emitted ultraviolet light to be absorbed in the upper electrode.
The present invention has been made in view of the above problem, and it is an object of the present invention to provide a semiconductor device, a display device, and an electronic appliance each having a light-emitting element with excellent luminous efficiency from a top surface. Moreover, it is another object of the present invention to provide a manufacturing method of a semiconductor device.